1. Field of the Invention
The present invention relates to a burner for combustion of fuel in pellet or granular form, and may be referred to as a “pellet burner” or “pellet furnace”. The invention is especially designed to operate with pellet or granular fuel of high ash content, for example grass or hay pellets. While the invention is particularly described in relation to a water heater or boiler, the invention may equally be applied to other combustion devices, e.g. a stove or space heater.
2. Prior Art
Pellet burners are known both for space heating and water heating. Usually, the pellets used are formed from wood, e.g. sawdust, and have an ash content usually less than 1%. When such pellets are burnt the ash is very light, and the combustion air fed to the burner, which is usually forced air provided by a fan, blows the ashes out of the top of the combustion chamber through a chimney.
It would be desirable to use pellets from other sources, e.g. made from plant material which is otherwise waste. Such materials however usually have an ash content of over 1%. For example, grass or hay pellets usually have over 5% ash content, and may be contaminated with sand and potassium chloride which produce glassy materials, in the form of clinkers or slag, when it burns. Such combustion products, which are usually heavier than the light hay or grass pellets, build up on a conventional burner grate and may put out the fire after a few hours. The ashes would have to be cleaned out too often for such operation to be practical.
It has been proposed, for example in U.S. Pat. No. 6,336,449 to Drisdelle et al., to make a pellet burner with a special combustion chamber, and restricted air supply, and it is claimed that this burner is capable of efficiently burning fuel containing 10% ash without the formation of slag and clinkers. This patent also suggests the use of a special grate; this has no moving parts.
U.S. Pat. No. 5,893,358 and U.S. Patent Application No. 2007/0125282 both show pellet burners in which clinkers or slag are periodically removed by reciprocating means associated with the grate.